We have found that two strains, H and HL of Ehrlich ascites tumor cells differ in their glycogen metabolism. The H cells contain glycogen but the HL cell does not. The H strain forms glycogen from glucose in vitro, but the HL strain does not. Both strains contain the full complement of enzymes of glycogen metabolism. (See Katz et al, Biochem. Biophys. Res. Comm., March, 1975.) We plan to study why HL cells form no glycogen. 1) The protein kinase and phosphatase interconverting the D and I forms of glycogen synthase will be assayed. 2) Cyclic AMP level and adenyl cyclase in the H and HL cells will be determined. 3) It will be seen whether HL cell extract contains a factor inhibiting glycogen synthesis. 4) The glycogen in H cells is mainly nuclear. Nuclei, microsomes and cytosol of H and HL cells will be isolated and assayed for glycogen synthase and phosphorylase.